1. Field of the Invention
The present invention relates to the field of microcontroller systems, more particularly reset methods and apparatus therefor.
2. Prior Art
The reset input of the Motorola 68HC11/16 microcontroller is a bi-directional input/output pin. It can be driven low externally to reset the microcontroller, or the microcontroller itself can drive it low, e.g., an internal watchdog timer fault. Typically, a number of other devices that may require resetting are also connected to this bus, a shown in FIG. 1.
Because the reset pin of the microcontroller is bi-directional, there are unique timing requirements associated with the change of state of the pin. Specifically, after a microcontroller generated reset, the microcontroller will release the reset output, wait 4 E cycles, and poll the state of the pin. This is done to ensure that no other device that is connected to the reset bus is asserting a reset. The factory recommended typical application suggests a 4.7 Kohm resistor connected between V.sub.CC and the reset pin to provide a pull-up current. Using this approach, the maximum capacitance allowed on the line is limited. For example, if E clock is 6 Mhz, 4 E cycles take 666 ns. The bus requires 1.6 RC time constants to reach the 0.8 V.sub.CC, which is V.sub.ih for the reset pin. This implies that a maximum of 88.6 pF is allowed on the bus. If upon polling the reset line after the 4 E cycles the microcontroller finds the line to still be low, the microcontroller interprets the apparent low state to mean that the reset signal was externally generated, and accordingly will go through a more involved reset procedure.
It is thus desirable to have the reset line return to an apparent high state within 4 E cycles after a microcontroller initiated reset to avoid the unnecessary, more time consuming reset sequence associated with an externally (non-microcontroller) initiated reset operation.